This invention relates to a process for regenerating a spent zeolite composite which involves contacting the composite with a caustic solution followed by calcination.
Zeolites have been used in a number of commercial processes for many years. With use these zeolites lose activity through various deactivation mechanisms. For example in adsorption processes contaminants such as hydrocarbons can be deposited onto the zeolite which over time become carbonaceous deposits. When these carbonaceous deposits reach a certain level, the zeolite can no longer function effectively and must be changed. The spent zeolite must be disposed of properly in compliance with EPA regulations.
An alternative to disposal is regeneration of the zeolite. The art discloses techniques for removing coke and/or other contaminants from zeolite compositions. One common regeneration technique is to burn the coke from the zeolite. However, applicant has found that merely burning coke off does not result in a zeolite with comparable performance to a fresh zeolite.
Other regeneration techniques include the use of steam or other solutions in combination with heating or calcining. For example, U.S. Pat. No. 5,093,293 discloses the use of steam for removing coke and other contaminants from Zeolite L. In U.S. Pat. No. 4,139,433 it is disclosed that a hydrocracking catalyst containing a Group VIII metal is regenerated by treating the spent catalyst with an ammonium hydroxide solution followed by calcination at 500xc2x0 F. to 950xc2x0 F. It is stated that the process redistributes the Group VIII metals and removes mono and divalent metal cations.
U.S. Pat. No. 4,975,399 discloses a two-step heating process to remove carbonaceous .deposits from a hydrotreating catalyst. U.S. Pat. No. 4,550,009 discloses treating a spent catalyst with a source of alkali or alkaline earth metal cations or ammonia and then extracting extractable nitrogen compounds with a liquid organic solvent.
In contrast to this art, applicant has developed a process for regeneration of a zeolite (used as an adsorbent) which involves contacting the spent zeolite with a caustic solution followed by calcination. The caustic is sodium hydroxide or potassium hydroxide. The combination of the two steps removes the carbon residue and anneals the zeolite so that it has adsorption properties substantially the same as the fresh zeolite.
As stated, this invention relates to a process for regenerating a spent composite consisting essentially of a crystalline zeolite and a binder, the process comprising contacting the spent composite with an aqueous caustic solution at a temperature of about 20xc2x0 C. to about 110xc2x0 C. for a time of about 1 to about 48 hours, separating the composite from the caustic solution, washing the composite with water, drying the composite and calcining the dried composite at a temperature of about 500xc2x0 C. to about 700xc2x0 C. for a time of about 1 to about 24 hours.
This and other objects and embodiments of the invention will become more apparent after a detailed description of the invention.
The composites which can be treated according to the instant process are any of those which only contain a zeolite and a binder. The zeolites which form the active part of the composite include but are not limited to Zeolite A, Zeolite Y, Zeolite X, mordenite, Zeolite beta and zeolites having the MFI structure, e.g., ZSM-5. The binders which are used in preparing the composites include clays, silica, alumina, and mixtures thereof. Specific examples of clays include attapulgite, bentonite, sepiolite, halloysite, and kaolinite. The zeolite and binder can be combined in various ratios but usually the binder is present from about 10 to about 90 wt. % of the composite.
The composite can be formed into various shapes by means well known in the art. Generally the zeolite and binder are combined along with water and optionally one or more additives selected from extrusion aids, dispersion aids, porosity modifiers, peptizing agents, etc. Examples of these additives are carboxymethylcellulose (extrusion aid), sodium salt of polyacrylic acid (dispersion aid), polyethylene (porosity modifier), nitric acid (peptizing agent). The zeolite, water and optional additive are homogeneously mixed by mulling, kneading, etc. Once a homogeneous mixture is obtained it is formed into shapes such as extrudates, pellets, pills, beads, etc., by means well known in the art. These shaped composites will possess the physical and chemical properties necessary for the intended use. For example, crush strength, attrition resistance, surface area, adsorption capacity, etc.
These composites are used in various adsorption processes where it is desirable to adsorb or separate one molecular species from another. One such process, which will be used to exemplify the invention, is the drying of natural gas. It should be pointed out that although composites that are used in drying of natural gas will be used as an example, the invention is not limited to those composites or the drying process. During the drying process (which uses the sodium form of Zeolite A as the zeolite component), the composite will also adsorb hydrocarbons such as hexanes, benzene, etc. When the composites contain from about 1 to about 10 wt. % carbon, usually about 5 wt. %, the zeolite has lost considerable adsorption capacity such that it is no longer effective in removing water from the natural gas. At this point the zeolite A containing composite is changed for fresh composite and the spent composite must be disposed of according to EPA regulations and procedures or regenerated according to the instant process.
The spent composites are regenerated according to the instant process by first contacting the composite with a caustic solution. The caustic solution is an aqueous solution with the caustic being sodium hydroxide, potassium hydroxide or mixtures thereof. It should be pointed out that by caustic solution is meant a strong base. Therefore, ammonium hydroxide which is defined as a weak base is not included in the definition of caustic. Additionally, ammonium hydroxide will exchange the alkali cation usually present in the zeolite and in some cases, e.g., zeolite A, will even destabilize the zeolite. The concentration of the strong base or caustic can vary from about 0.1N to about 4.0N. Prior to contacting the composite with the caustic solution one can optionally crush the composite to obtain a powder which can facilitate the contacting. Another optional step is to first heat the composite in air at a temperature of about 200xc2x0 C. to about 800xc2x0 C. and preferably in a range of about 400xc2x0 C. to about 700xc2x0 C. in order to remove carbonaceous deposits including volatile organic compounds.
The composite and caustic solution are contacted in a batch mode. The relative amount of composite which is added to the caustic solution can vary considerably but is usually that amount that will give a slurry which contains from about 5 to about 25 wt. % composite. The caustic solution and composite are contacted at treating conditions which include a time of about 1 to about 48 hours at a temperature of about 20xc2x0 C. to about 110xc2x0 C. Preferably the contacting time is from about 2 to about 24 hours with the longer times at the lower temperatures.
Once the contacting is complete, the solids are filtered from the caustic solution and the solids are washed with water to remove any residual caustic. The washed composite is now dried at a temperature of ambient temperature (i.e., about 20xc2x0 C.) to about 200xc2x0 C. for a time of about 1 to about 24 hours and then calcined at a temperature of about 500xc2x0 C. to about 700xc2x0 C. for a time of about 1 to 24 hours. Of course it is understood that the drying and calcining steps can be carried out in one step by using belt furnaces, rotary kilns and the like.
The regenerated composite is characterized in that it has at least 90% and preferably at least 95% of the key physical and chemical properties of the fresh composite. That is, if the key property for the particular application is water adsorption (i.e., for drying applications) then the regenerated zeolite composite should have at least 90% of the water adsorption of the fresh zeolite composite. Of course even though the other characteristics may not reach at least 90% of the fresh values, they should be at least 80% of the fresh values. If the degree of regeneration, i.e., percent of fresh performance, is not obtained in one step, then the procedure described above can be repeated one or more times.
In the case where the composite has been crushed into a powder, an additional step, after the drying step is to reform the crushed composite into a shaped article. This can be done by means well known in the art as described above. Namely, the powder is mixed with water and optionally additives to form a mass which can be formed into various shapes such as extrudates. If desired, additional binder may be added to the mixture. Once the shaped articles are formed, they are calcined at a temperature of about 500xc2x0 C. to about 700xc2x0 C.
Without wishing to be bound by any particular theory, it appears that the caustic serves a dual purpose. The high pH of the caustic solution provides adequate hydroxyls (OHxe2x88x92) to rehydroxylate the binder which was dehydroxylated during the initial calcination step. The rehydroxylation may release or xe2x80x9cdecomplexxe2x80x9d the carbonaceous residue on the composite such that the carbonaceous residue is more easily burned off during the subsequent calcination. Additionally, it is hypothesized that the presence of the alkali metals in the caustic solution enables the reinsertion of non-framework aluminum atoms back into the framework thus affecting a xe2x80x9chealingxe2x80x9d of the damaged zeolite crystal and improving adsorption capacities dramatically.